Power conversion refers to the conversion of one form of electrical power to another desired form and voltage, for example converting 115 or 230 volt alternating current (AC) supplied by a utility company to a regulated lower voltage direct current (DC) for electronic devices, referred to as AC-to-DC power conversion.
A switched-mode power supply, switching-mode power supply or SMPS, is a power supply that incorporates a switching regulator. While a linear regulator uses a transistor biased in its active region to specify an output voltage, an SMPS actively switches a transistor between full saturation and full cutoff at a high rate. The resulting rectangular waveform is then passed through a low-pass filter, typically an inductor and capacitor (LC) circuit, to achieve an approximated output voltage. The switch mode power supply uses the high frequency switch, the transistor, with varying duty cycle to maintain the output voltage. The output voltage variations caused by the switching are filtered out by the LC filter.
An SMPS can provide a step-up, step-down or inverted output voltage function. An SMPS converts an input voltage level to another level by storing the input energy temporarily and then releasing the energy to the output at a different voltage. The storage may be in either electromagnetic components, such as inductors and/or transformers, or electrostatic components, such as capacitors.
Advantages of the SMPS over the linear power supply include smaller size, better power efficiency, and lower heat generation. Disadvantages include the fact that SMPSs are generally more complex than linear power supplies, generate high-frequency electrical noise that may need to be carefully suppressed, and have a characteristic ripple voltage at the switching frequency.
High-frequency ripple results when passing current through the transistor switches and then filtering the current with passive components. The frequency components of the ripple are dependent on both the switching frequency and the switching speeds of the semiconductor switches. The high-frequency ripple generates unwanted electromagnetic interference (EMI) and must be removed to a high degree for the converter to pass standard EMI requirements.
Conventional power converters pass EMI requirements by reducing the input and output ripple. Reduction is accomplished by the following methods: large filters, reduction of switching frequency, and/or reduction of switching speeds. Such techniques are commonly practiced in nearly all conventional power converters. However, use of each of these techniques comes with specific drawbacks. Use of large filters adds space and cost. Reduction of switching frequency increases the size of passive components and cost. Reduction of switching speeds reduces efficiency.
The power factor of an AC electric power system is defined as the ratio of the real power to the apparent power, and is a number between 0 and 1. Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can be greater than the real power. Low-power-factor loads increase losses in a power distribution system and result in increased energy costs. Power factor correction (PFC) is a technique of counteracting the undesirable effects of electric loads that create a power factor that is less than 1. Power factor correction attempts to adjust the power factor to unity (1.00).
High power applications, and some low power applications, require the converter to draw current from the AC line with a high power factor. Boost converters are commonly used to produce the high power factor input. A bridge rectifier is commonly connected to an input AC voltage for converting the input AC voltage into a full-wave rectified DC voltage before the voltage is stepped-up. However, the rectifying diodes that constitute the bridge rectifier cause considerable conduction loss resulting in power conversion efficiency degradation. As such, conventional PFC boost converters that include a bridge rectifier typically fail to provide sufficient efficiency for high power applications.
PFC boost converters that do not include a bridge rectifier, commonly referred to as bridgeless PFC boost converters, provide improved efficiency and reduced conduction loss compared to similar PFC boost converters having a bridge rectifier. FIG. 1 illustrates a circuit diagram of a conventional bridgeless power factor correction boost converter. In FIG. 1, a boost inductor L11 is coupled to a first node of an input AC voltage Vin, and a boost inductor L12 is coupled to a second node of the input AC voltage Vin. A transistor switch S11 is coupled to the boost inductor L11, and a transistor switch S12 is coupled to the boost inductor L12. A rectifying diode D11 is coupled to the boost inductor L11 and also in series with the transistor switch S11. A rectifying diode D12 is coupled to the boost inductor L12 and also in series with the transistor switch S12. The rectifying diodes D11, D12 are coupled to an output capacitor C11 through a first bus and the transistor switches S11, S12 are coupled to the output capacitor C11 through a second bus. The output capacitor C11 is coupled to a load R11.
During a positive half-cycle of the input AC voltage Vin, the transistor switch S11 is turned on and an input current is induced to flow toward the boost inductor L11 so as to charge the boost inductor L11. Concurrently with the transistor switch S11 turned on, the transistor switch S12 is also turned on and the current path is closed through the body diode of the transistor switch S12. Next, still during the positive half-cycle of the input AC voltage, the transistor switch S11 is turned off and the energy stored in the boost inductor L11 is discharged to the output capacitor C11 through the rectifying diode D11. The current path is closed through the body diode of the transistor switch S12, where the current path is from the input AC voltage Vin, through the boost inductor L11, through the rectifying diode D11, through the output capacitor C11, through the body diode of the transistor switch S12, through the boost inductor L12 and back to the input AC voltage Vin.
During the negative half-cycle of the input AC voltage Vin, the transistor switch S12 is turned on and an input current is induced to flow toward the boost inductor L12 so as to charge the boost inductor L12. Concurrently with the transistor switch S12 turned on, the transistor switch S11 is also turned on and the current path is closed through the body diode of the transistor switch S11. Next, still during the negative half-cycle of the input AC voltage Vin, the transistor switch S12 is turned off and the energy stored in the boost inductor L12 is discharged to the output capacitor C11 through the rectifying diode D12. The current path is closed through the body diode of the transistor switch S 11. As such, during each half-cycle of the input AC voltage Vin, one transistor switch functions as an active switch and the other transistor switch functions as a rectifying diode. A disadvantage of the converter shown in FIG. 1 is that output voltage value floats compared to the input AC voltage Vin and ground. Another disadvantage is that the converter of FIG. 1 suffers from a severe EMI noise problem due in part to the increase of the parasitical capacitance value between the buses and ground.
FIG. 2 illustrates a circuit diagram of another conventional bridgeless power factor correction boost converter. The bridgeless power factor correction boost converter of FIG. 2 is a modified circuit diagram of the bridgeless power factor correction boost converter of FIG. 1. The boost inductors L21, L22, the transistor switches S21, S22, the rectifying diodes D21, D22, and the output capacitor C21 of FIG. 2 are configured and operate similarly to the configuration and operation of the of the boost inductors L11, L12, the transistor switches S11, S12, the rectifying diodes D11, D12, and the output capacitor C11, respectively, of FIG. 1. The bridgeless PFC boost converter of FIG. 2 adds a pair of auxiliary diodes D23, D24 to the input side of the converter to more efficiently suppress the EMI noise of the converter. However, power loss across the diodes D23, D24 lowers the circuit efficiency.